Generally known principles of suppressing NOx emissions include the suppression of flame (combustion gas) temperatures and a decrease in retention time of combustion gas at high-temperatures. As such, various technologies are available for decreasing the emission of nitrogen oxides by applying these principles. Various methods have been proposed and put into practical use, for example, two-stage combustion, lean-rich combustion, exhaust gas recirculate combustion, water mixing combustion, steam injection combustion, and flame cooling combustion by a water tube group.
Moreover, NOx sources relatively small in capacity such as water-tube boilers are also beginning to be required for a further decrease in emission of NOx due to an increasing awareness of environmental problems. In this case, the decrease in NOx generation inevitably entails an increased amount of emitted CO, thus making it difficult to attain a simultaneous decrease in NOx and CO.
A cause of the above problem is that a simultaneous decrease in emission of NOx and CO is technically incompatible. More specifically, when temperatures of combustion gas are abruptly lowered and kept at temperatures of 900° C. or less in an attempt to decrease the emission of NOx to result in an ample generation of CO, the thus generated CO is emitted before oxidization to increase the amount of emitted CO. In other words, temperatures of combustion gas are kept higher in an attempt to decrease the amount of emitted CO, thus resulting in an insufficient suppression of NOx generation.
In order to solve the above problem, the applicant has proposed low NOx and low CO emission technologies for decreasing as much as possible the amount of CO, which is generated in accordance with a decrease in the amount of NOx generation, and also suppressing temperatures of combustion gas so as to attain oxidation of the thus generated CO. The technologies are now commercially feasible (refer to Patent Documents 1 and 2). However, an actual value of emitted NOx remains to be about 25 ppm in the low NOx emission technologies described in Patent Documents 1 and 2.
In order to solve the above problem, the applicant has proposed a low NOx combustion method in which a NOx decreasing step is conducted to suppress temperatures of combustion gas so as to give priority to suppression of NOx generation rather, than a decrease in the amount of emitted CO, thereby keeping the value of the thus generated NOx to a predetermined value or lower, and a CO decreasing step is, thereafter, conducted so as to keep the value of CO emitted from the NOx decreasing step to a predetermined value or lower (refer to Patent Documents 3 and 4). The technologies disclosed in Patent Documents 3 and 4 are able to decrease the amount of emitted NOx to a value lower than 10 ppm, but find it difficult to decrease the amount of emitted NOx to a value below 5 ppm. This is due to the fact that combustion characteristics inevitably entail NOx generation at 5 ppm or greater.
Then, in the low NOx emission technologies disclosed in Patent Documents 3 and 4, as shown in FIG. 15, combustion is affected at a high air-ratio combustion region Z1 where the air ratio is 1.38 or greater. In contrast, at a combustion region Z2 where the air ratio is 1.1 or lower (hereinafter, referred to as “low air ratio”), nitrogen oxides are generated in an increased amount, thus making it difficult to attain a simultaneous decrease in the amount of emitted NOx and CO. There is also posed a difficulty in controlling a stable combustion due to a possible occurrence of backfire where the air ratio is 1 or lower. Therefore, the low air-ratio combustion region Z2 has hardly been subjected to research and development. In FIG. 15, the lines F and E graphically show NOx characteristics and CO characteristics on a primary side of a combustion apparatus of the present invention, and the lines U and J graphically show NOx characteristics and CO characteristics of the combustion apparatus of the present invention. Both of the low NOx emission technologies on a secondary side disclosed in Patent Documents 3 and 4 are in principle those in which a burner is used to conduct combustion at the high air ratio region Z1, thereby suppressing the generation of NOx and removing the thus generated CO through an oxidation catalyst (Patent Documents 3 and 4).
On the other hand, there is a growing demand for operating boilers at a low air ratio not only to attain a greater decrease in emitted NOx but also to save energy.
The inventors of the present application have been engaged in research and development of a combustion method of decreasing the amount of emitted nitrogen oxides to zero as much as possible by use of an oxidation catalyst.
Moreover, the method disclosed in Patent Document 5 is known as that of treating nitrogen oxide-containing gas generated on combustion by a burner.
According to the method of treating exhaust gas in Patent Document 5, a burner is used to conduct combustion at an air ratio lower than 1.0, whereby oxygen is not contained in combustion exhaust gas but unburned components such as CO and HC (hydrocarbons) are contained, and a nitrogen oxide reducing catalyst is used to reduce nitrogen oxides by unburned components, thereby purifying the nitrogen oxides. Then, air is supplied to exhaust gas after purification, thereby purifying the unburned components by using an oxidation catalyst.
The treatment method of Patent Document 5 is not a method of decreasing carbon monoxide and nitrogen oxides in the presence of oxygen. Further, according to the method described in Patent Document 5, unburned hydrocarbons are emitted in a great amount, thus making it difficult to decrease the concentrations of emitted nitrogen oxides and emitted carbon monoxide to substantially zero by using an oxidation catalyst. Further, an oxidation catalyst having characteristics in which the reduction efficiency of nitrogen oxides decreases in the presence of hydrocarbon cannot be used. Still further, in a step of reducing nitrogen oxides, a catalyst is used, which is different from that used in a step of oxidizing unburned components, resulting in a complicated treatment.
Further, a method of purifying nitrogen oxide-containing gas emitted from a gas engine is known in Patent Document 6. Patent Document 6 describes that nitrogen oxides and carbon monoxide are purified by using a three-way catalyst, which essentially requires the presence of hydrocarbons in gas and is applicable only to gas at a theoretical air ratio in which no excess oxygen is present. Therefore, the treatment method of Patent Document 6 is not suitable in treating combustion gas resulting from a combustion apparatus such as a boiler, which occurs on combustion by a burner and contains excess oxygen.
Still further, a technology in which an oxidation catalyst is used to reduce nitrogen oxides contained in exhaust gas derived from an incinerator by carbon monoxide is known in Patent Document 7. According to the technology of Patent Document 7, since nitrogen oxides is not reduced in the presence of oxygen in exhaust gas, fuel is burned at an excessively high concentration (air ratio of less than 1) on primary combustion, by which exhaust gas is kept deprived of oxygen. The technology in Patent Document 7 is subjected to such restriction that fuel is burned at an excessively high concentration, thus making it difficult to find an application for the combustion apparatus such as a burner-equipped boiler in which oxygen is contained in exhaust gas.    [Patent Document 1] Japanese Patent No. 3221582    [Patent Document 2] U.S. Pat. No. 5,353,748    [Patent Document 3] Japanese unexamined Patent Application, First Publication No. 2004-125378    [Patent Document 4] U.S. Pat. No. 6,792,895    [Patent Document 5] Japanese unexamined Patent Application, First Publication No. 2001-241619    [Patent Document 6] Japanese unexamined Patent Application, First Patent Document 5-38421    [Patent Document 7] Japanese unexamined Patent Application, First Publication No. 2003-275543